Gas separation processes with membranes have undergone a major evolution since the introduction of the first membrane-based industrial hydrogen separation process about two decades ago. The design of new materials and efficient methods will further advance the membrane gas separation processes within the next decade.
The gas transport properties of many glassy and rubbery polymers have been measured, driven by the search for materials with high permeability and high selectivity for potential use as gas separation membranes. Unfortunately, an important limitation in the development of new membranes for gas separation applications is a well-known trade-off between permeability and selectivity. By comparing the data of hundreds of different polymers, Robeson demonstrated that selectivity and permeability seem to be inseparably linked to one another, in a relation where selectivity increases as permeability decreases and vice versa.
Despite concentrated efforts to tailor polymer structure to improve separation properties, current polymeric membrane materials have seemingly reached a limit in the tradeoff between productivity and selectivity. For example, many polyimide and polyetherimide glassy polymers such as Ultem 1000 have much higher intrinsic CO2/CH4 selectivities (αCO2/CH4) (˜30 at 50° C. and 100 psig) than that of cellulose acetate (˜22), which are more attractive for practical gas separation applications. These polymers, however, do not have outstanding permeabilities attractive for commercialization compared to current commercial cellulose acetate membrane products, in agreement with the trade-off relationship reported by Robeson.
To enhance membrane selectivity and permeability, mixed matrix membranes (MMMs) have been developed in recent years. To date, almost all of the MMMs reported in the literature are hybrid blend membranes comprising insoluble solid domains such as molecular sieves or carbon molecular sieves embedded in a polymer matrix. For example, see U.S. Pat. No. 6,626,980; US 2003/0220188 A1; US 2005/0043167 A1; US 2002/0053284 A1; U.S. Pat. No. 6,755,900; U.S. Pat. No. 6,500,233; U.S. Pat. No. 6,503,295 and U.S. Pat. No. 6,508,860. These MMMs combine the low cost and easy processability of the polymer with the superior gas separation properties provided by the molecular sieve. These membranes have the potential to achieve higher selectivity with equal or greater permeability compared to existing polymer membranes, while maintaining their advantages. In contrast to the many studies on conventional polymers for membranes, only a few attempts to increase gas separation membrane performance with MMMs of zeolite and rubbery or glassy polymers have been reported. These MMMs have shown some promise, but there remains a need for improved membranes that combine the desired higher selectivity and permeability goals previously discussed.
In the present invention, it has been found that a new type of metal-organic framework (MOF)-polymer or metal-organic polyhedra (MOP)-polymer MMM achieves significantly enhanced gas separation performance (higher αCO2/CH4) compared to that of cellulose acetate membranes. MOFs such as isoreticular MOF-5 (IRMOF-1) as the dispersed filler phase in MMMs using glassy polymer such as Matrimid 5218 as the continuous polymer matrix have been prepared and the membrane properties determined.
These MOFs and similar structures were recently reported. Simard et al. reported the synthesis of an “organic zeolite”, in which rigid organic units are assembled into a microporous, crystalline structure by hydrogen bonds. See Simard et al., J. AM. CHEM. SOC., 113:4696 (1991). Yaghi and co-workers and others have reported a new type of highly porous crystalline zeolite-like materials termed “metal-organic frameworks” (MOFs). These MOFs are composed of ordered arrays of rigid organic units connected to metal centers by metal-ligand bonds and they possess vast accessible surface areas. See Yaghi et al., SCIENCE, 295: 469 (2002). MOF-5 is a prototype of a new class of porous materials constructed from octahedral Zn—O—C clusters and benzene links. Most recently, Yaghi et al. reported the systematic design and construction of a series of frameworks (IRMOF) that have structures based on the skeleton of MOF-5, wherein the pore functionality and size have been varied without changing the original cubic topology. For example, IRMOF-1 (Zn4O(R1—BDC)3) has the same topology as that of MOF-5, but was synthesized by a simplified method. In 2001, Yaghi et al. reported the synthesis of a porous metal-organic polyhedron (MOP) Cu24(m-BDC)24(DMF)14(H2O)50(DMF)6(C2H5OH)6, termed “α-MOP-1” and constructed from 12 paddle-wheel units bridged by m-BDC to give a large metal-carboxylate polyhedron. These MOF, IR-MOF and MOP materials exhibit analogous behaviour to that of conventional microporous materials such as large and accessible surface areas, interconnected intrinsic micropores. Moreover, they also can possibly reduce the hydrocarbon fouling problem of the polyimide membranes due to the presence of pore sizes larger than those of zeolite materials. MOF, IR-MOF and MOP materials are also expected to allow the polymer to infiltrate the pores, which would improve the interfacial and mechanical properties and would in turn affect permeability. These MOF, IR-MOF and MOP materials are selected as the fillers in the preparation of new MMMs in this invention.